Isolation products are used to provide electrical isolation to transfer signals between circuits of different voltage levels and/or between circuits that operate from different supply voltages and ground references. One type of isolator is called an opto-isolator or opto-coupler that provides an optical transmission path to transfer signals between circuits that can be electrically isolated from one another. Opto isolators are used in data communication, power supplies, and other systems where high voltage or high slew-rate common-mode signals appear between the input and output. Opto-isolators are often constructed as a transmitter and a receiver separated by glass and packaged together. Glass provides dc electrical isolation and optical transmission, but suffers from capacitive coupling between the transmitter and receiver and also adds cost to the device. Transformers provide isolation between magnetically coupled primary and secondary coils separated by an insulator to transfer data and/or power between two circuits that are separated by a high voltage or high slew-rate common-mode signal. Some transformers include a magnetic coupling material to enhance the magnetic coupling between the primary and secondary.